Method and apparatus for the fabrication of ferroelectric films

ABSTRACT

The present invention is related to methods and apparatus for processing weak ferroelectric films on semiconductor substrates, including relatively large substrates, e.g., with 300 millimeter diameter. A ferroelectric film of zinc oxide (ZnO) doped with lithium (Li) and/or magnesium (Mg) is deposited on a substrate in a plasma assisted chemical vapor deposition process such as an electron cyclotron resonance chemical vapor deposition (ECR CVD) process. Zinc is introduced to a chamber through a zinc precursor in a vaporizer. Microwave energy ionizes zinc and oxygen in the chamber to a plasma, which is directed to the substrate with a relatively strong field. Electrically biased control grids control a rate of deposition of the plasma. The control grids also provide Li and/or Mg dopants for the ZnO to create the ferroelectric film. A desired ferroelectric property of the ferroelectric film can be tailored by selecting an appropriate composition of the control grids.

RELATED APPLICATION

[0001] This application is a continuation application of U.S.application Ser. No. 09/810,368, entitled “METHOD AND APPARATUS FOR THEFABRICATION OF FERROELECTRIC FILMS,” filed Mar. 15, 2001.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention is generally related to semiconductorprocessing. In particular, the present invention relates to processingferroelectric films.

[0004] 2. Description of the Related Art

[0005] A ferroelectric material is a material that exhibits an abilityto maintain an electric polarization in the absence of an appliedelectric field. Ferroelectric materials also exhibit piezoelectricity,where the material changes polarization in response to a mechanicalpressure or strain, and pyroelectricity, where the material changespolarization in response to a temperature change.

[0006] The foregoing properties of ferroelectric materials have led tomany practical applications. One application uses the ability of theferroelectric material to retain a polarization state to store data in anon-volatile memory device.

[0007] A film of zinc oxide (ZnO) doped with lithium (Li) and/ormagnesium (Mg) is known to form a weak ferroelectric film, AkiraOnodera, et al., Ferroelectric Properties in Piezoelectric SemiconductorZ _(n1)-xM _(x) O (M=Li, Mg), 36 Japan J. Appl. Phys. 6008 (1997). In aprior patent application, Applicants disclosed a non-volatilesemiconductor memory fabricated from a doped ZnO film, WeakFerroelectric Memory Transistor, application Ser. No. 09/383,726, filedAug. 26, 1999, the entirety of which is hereby incorporated byreference.

[0008] ZnO in stoichiometric form is an electrical insulator.Conventional methods of doping host ZnO with Li and/or Mg to formferroelectric films have proven inadequate. Conventional methods are notwell suited to the doping of host ZnO with Li and/or Mg for relativelylarge scale operations with wafers of approximately 300 millimeters(about 12 inches) or larger.

[0009] Magnetron sputtering is a conventional method of doping ZnO withLi and/or Mg. In magnetron sputtering, a target produced from acomposition of ZnO and Li and/or Mg is introduced into a sputteringsystem. The composition can be made from ZnO with strips or particles ofLi and /or Mg. Powder metallurgy can also be used to create the target.

[0010] A magnetron sputtering system creates a plasma, which reacts withthe surface of the target to create the film. Disadvantageously, thefilm composition cannot be fine-tuned because the doping levels of Liand/or Mg are dictated by the initial composition of the target. Anotherdisadvantage of magnetron sputtering is that relatively large targets,such as 300-millimeter targets, are relatively difficult to produceusing powder metallurgy. A further disadvantage of magnetron sputteringwith powder metallurgy is that the purity of ZnO in a powdered metaltarget process is relatively lower than the purity of ZnO that isattainable from a zone-refined process.

[0011] Jet vapor deposition (JVD) is another conventional method offorming a ZnO film (with or without doping of Li and/or Mg) on asubstrate. In a JVD process, jets of a light carrier gas, such ashelium, transport the depositing vapor of ZnO to the substrate.Uniformity of the thickness of the deposited film can require the JVDprocess to move and rotate the substrate relative to the jet nozzles ina complex mechanical motion. The chamber performing the JVD process canquickly grow to relatively large and expensive proportions as thechamber holding the substrate should be at least twice the diameter ofthe substrate wafer to accommodate the complex mechanical motion.Further, a JVD process does not permit the tailoring of the Li and/or Mgdoping of the ZnO to conform the ZnO film to a desired ferroelectriccharacteristic.

[0012] Low-pressure chemical vapor deposition (LP-CVD) is still anotherconventional method of growing a ZnO film, with or without Li and/or Mgdoping, on a substrate. As with the above-noted processes, the LP-CVDprocess also does not permit the tailoring of the Li and/or Mg doping ofthe ZnO to conform the ZnO film to a desired ferroelectriccharacteristic is not easy.

[0013] Thus, conventional methods are not well adapted to produceferroelectric films on large wafers. The processing of ferroelectricfilms on large wafers of substrate can dramatically reduce a per-unitcost of chips made from the wafers. Conventional methods are also notwell adapted to permit the uniform tailoring of the deposited ZnOcomposition to permit the tailoring of the ZnO film to a desiredferroelectric characteristic.

SUMMARY OF THE INVENTION

[0014] The present invention is related to methods and apparatus forprocessing weak ferroelectric films on semiconductor substrates. Aferroelectric film of zinc oxide (ZnO) doped with lithium (Li) and/ormagnesium (Mg) is deposited on a substrate in an electron cyclotronresonance chemical vapor deposition (ECR CVD) process. An embodimentaccording to the invention advantageously permits the fabrication ofrelatively large (about 300 millimeters in diameter) substrates, whichallows more devices to be fabricated at the same time, i.e., enhanceseconomies of scale.

[0015] In accordance with one embodiment of the present invention, thezinc is introduced to a chamber through a zinc precursor in a vaporizer.Argon gas transports the zinc to the chamber. Microwave energy ionizeszinc and oxygen in the chamber to a plasma, which is directed to thesubstrate with a relatively strong magnetic field. Electrically biasedcontrol grids provide a control of a rate of deposition of the plasma onthe substrate. The control grids also provide Li and/or Mg dopants forthe ZnO to create the ferroelectric film. The properties of theferroelectric film can be conveniently tailored by selecting thecomposition, height, spacing and/or applied voltage of the control grid,which acts as a source of dopant, e.g., Li and/or Mg, for the ZnO film.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] These and other features of the invention will now be describedwith reference to the drawings summarized below. These drawings and theassociated description are provided to illustrate preferred embodimentsof the invention, and not to limit the scope of the invention.

[0017]FIG. 1 is an electron cyclotron resonance chemical vapordeposition system according to an embodiment of the present invention.

[0018]FIG. 2 is a top-view of a wire screens.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] Although this invention will be described in terms of certainpreferred embodiments, other embodiments that are apparent to those ofordinary skill in the art, including embodiments which do not provideall of the benefits and features set forth herein, are also within thescope of this invention. Accordingly, the scope of the present inventionis defined only by reference to the appended claims.

[0020] An embodiment according to the invention includes a plasmaenhanced or assisted chemical vapor deposition system, moreparticularly, an electron cyclotron resonance chemical vapor deposition(ECR CVD) system, which uniformly deposits a thin film of ferroelectricmaterial having a controlled doping level. In the illustratedembodiment, the material comprises zinc oxide (ZnO) with controlledamounts of metal doping, specifically lithium (Li) and/or magnesium(Mg), on a relatively large wafer. The production of the ferroelectricfilm on the relatively large wafer can result in a dramatic reduction toproduction costs as more devices can be formed at a time. For example,if a diameter of a given wafer doubles, the surface area of the waferquadruples and can thereby accommodate approximately four times thenumber of devices. The ECR CVD system can control the doping of the Liand/or Mg such that the ferroelectric characteristics of the ZnO can betailored to a desired characteristic. The tailoring of the ferroelectriccharacteristics further enhances the yield of processed wafers.

[0021]FIG. 1 illustrates an ECR CVD system 100 (not to scale) accordingto one embodiment of the invention. The ECR CVD system is a plasmadeposition technique, which also includes plasma assisted CVD and plasmaenhanced CVD. The ECR CVD system 100 includes a deposition chamber 102,where a substrate 104 is processed. Desirably, the chamber 102 isconfigured to support a single wafer or substrate 104, in accordancewith state-of-the-art integrated circuit (IC) fabrication. In addition,the chamber 102 can be configured to support a small number ofsubstrates, e.g., less than 5 substrates, facing the deposition sources.Generally, the components the ECR CVD system 100 can be fabricated froma variety of materials including stainless steel. The deposition chamber102 can be formed within a cavity of fused quartz. Of course, the ECRCVD system 100 includes an access door or port to provide access to thedeposition chamber 102 for loading and unloading substrates.

[0022] The substrate 104 is placed on a substrate support, chuck or tray106, which rests on an insulator 108 that attaches to a stand 110. Thetray 106 is preferably slightly larger in diameter than the substrate104. For example, where the substrate 104 is 300 millimeters (mm) indiameter, the tray can be 35 centimeters (cm) in diameter. The tray 106,and thereby the substrate 104, can be connected to an external firstpower supply 112 through a first conductor 114. The tray 106 furtherprovides attachment for guides 116, which support a first grid 118 and asecond grid 120. The first and second grids 118, 120 are connected to asecond power supply 122 through a second conductor 124. The tray 106further includes a heat source 156.

[0023] The ECR CVD system 100 further includes a plasma generator in theform of an electromagnetic excitation chamber 126. Reactant source gasesare communicated to the excitation chamber 126 by reactant gas inlets128, which in the illustrated embodiment communicate with a vaporizer130. The electromagnetic excitation chamber 126 also communicates with amicrowave source 132, an upper magnet assembly 134, and a lower magnetassembly 136. The vaporizer 130 connects to at least one of the reactantgas inlets 128 via a first passage 138 and a mass flow controller orfirst valve 140. A second passage 142 and a second valve 144 coupleanother reactant gas inlet 128, or a manifold leading to a common inlet,to an oxygen source. In the illustrated ECR CVD system 100, themicrowave source 132 couples to the electromagnetic excitation chamber126 through a quartz window 146. The upper and lower magnet assemblies134, 136 can be attached to the electromagnetic excitation chamber 126to create a magnetic field therein. The dashed line 148 indicates thedivision between the deposition chamber 102 and the electromagneticexcitation chamber 126. The electromagnetic excitation chamber 126 isalso known as a microwave plasma chamber.

[0024] The operation of the ECR CVD system 100 will now be describedwith reference to FIG. 1.

[0025] After the substrate 104 is placed on the chuck or tray 106 andthe ECR CVD system 100 has been sealed, the deposition chamber 102 isbrought to a vacuum through vacuum pumps connected to evacuation outlets150. In one embodiment, the system is first evacuated to a base pressurebelow 2×10⁻⁶ Torr using a turbomolecular pump.

[0026] Argon or other carrier gas is introduced into the vaporizer 130through a vaporizer inlet 152. The vaporizer 130 contains a first sourcematerial 154, such as Zn(C₅H₇O₂ ⁻)₂ or Zn(acac)₂, where acac is acetylacetonate. Zn(C₅H₇O₂ ⁻)₂ is a precursor providing Zn. In one embodiment,the vaporizer 130 maintains the first source material 154 atapproximately 105 degrees Celsius (C.).

[0027] The argon gas carrier introduces the Zn precursor into theelectromagnetic excitation chamber 126 through the reactant gas inlet128. The first valve 140 controls the rate by which Zn is introducedinto the electromagnetic excitation chamber 126 by controlling the rateat which argon flows through the vaporizer 130. In one embodiment, argonflows through the vaporizer 130 at approximately 100 cubic centimetersper minute.

[0028] In the illustrated embodiment, a second reactant comprising asource of oxygen, such as diatomic oxygen (O₂) or ozone (O₃), is alsointroduced into the electromagnetic excitation chamber 126 via thereactant gas inlet 128. The second valve 144 controls the flow of oxygensource or precursor gas from the second passage 142, which leads to theoxygen source. In one embodiment, the oxygen precursor comprises O₂ andthe oxygen flow is approximately 2 cubic decimeters per minute. Varyingthe rate of gas flow and/or the gas flow ratio varies a rate ofdeposition of the dopants.

[0029] During deposition, the vacuum pumps and the first and the secondvalves 140, 144 maintain the pressure in the deposition chamber 102 at arelatively low vacuum, such as from 7 to 24 milliTorr (mTorr). Thepressure can vary widely, but higher pressures tend to weaken theelectron cyclotron resonance effect and reduce the density of the plasmagenerated by the microwave source 132. In one embodiment, the pressurein the deposition chamber 102 is maintained at 6 mTorr, although it canbe monitored and maintained between about 7 mTorr to about 25 mTorr.

[0030] The microwave source 132 generates relatively high-poweredmicrowaves, which are coupled to the electromagnetic excitation chamber126 through the quartz window 146. The microwave source 132 can beremotely located from the electromagnetic excitation chamber 126 andcoupled to the quartz window 146 with a waveguide. In one embodiment,the microwave source 132 is a magnetron that transmits approximately 300to 400 Watts of microwave power to the electromagnetic excitationchamber 126 at a frequency of approximately 2.45 gigahertz (GHz). Theenergy is coupled to the reactant gases, breaking down Zn and oxygenprecursors in the electromagnetic excitation chamber 126 to generateplasma ions and neutral radicals.

[0031] The upper magnet assembly 134 and the lower magnet assembly 136induce a relatively powerful magnetic field within the electromagneticexcitation chamber 126. The upper magnet assembly 134 and the lowermagnet assembly 136 can be fabricated from solenoids, where the windingsof the solenoids wrap around the electromagnetic excitation chamber 126as shown in FIG. 1. A magnetic flux density of approximately 875 Gaussproduced in the middle of the electromagnetic excitation chamber 126suffices to allow an electron cyclotron resonance.

[0032] Of course, the magnetic field produced by a solenoid can varywith the current and the number of turns of wire in the solenoid. In oneembodiment, the current drive to an upper solenoid embodying the uppermagnet assembly 134 is approximately 20% greater than a current drive toa lower solenoid embodying the lower magnet assembly 136 such that theupper solenoid produces a greater magnetic field than the lower solenoidand thereby accelerates or propels ions within the plasma toward thedeposition chamber 102. For example, the current drive to the upper andthe lower solenoids can conform to approximately 120 Amps and 100 Amps,respectively.

[0033] The ferroelectric film is deposited on the substrate 104 in thedeposition chamber 102. The substrate 104 rests on a tray 106, which ispreferably heated by the heat source 156. The tray 106 can be fabricatedfrom a material that is conductive to heat, such as a ceramic, such thatthe heat from the heat source 156 is relatively evenly distributed. Oneembodiment of a heat source 156 passes a current through a resistivewire, such as NiChrome. A third power supply 158 sources the currentthrough wires 160. The heat supplied can vary in accordance with thepower applied to the resistive wire by the third power supply 158. Ofcourse, the tray 106 can further include a temperature sensor, such as athermocouple or temperature sensitive resistor, to monitor and/orcontrol the temperature of the tray 106. In one embodiment, the heatsource 156 maintains the temperature of the tray 106 and the substrate104 within a range of approximately 350 degrees C. to 650 degrees C. Theskilled artisan will also appreciate that the substrate 104 can beotherwise heated (e.g., inductively or radiantly).

[0034] The rate at which ZnO is deposited on the substrate can becontrolled by a position, a dimension, and a bias of the first and thesecond grids 118, 120. The first and the second grids 118, 120 alsointroduce a second source material that is sputtered on the substrate.The second source material is a metal dopant, preferably Li and/or Mg,which combines with the dielectric ZnO to produce the ferroelectricfilm. Further details of a wire screen 200 that can form the first andthe second grids 118, 120 are described below in connection with FIG. 2.

[0035] The first and the second grids 118, 120 attach to the guides 116,which hold the first and the second grids 118, 120 above the substrate104. In one embodiment, the guides 116 are four insulating polesfabricated from an insulating polyimide polymer such as Vespel® from E.I. du Pont de Nemours and Company. The spacing between the first and thesecond grids 118, 120 and the spacing between the first grid 118 and thesubstrate 104 can be adjusted along the guides 116 to vary the rate ofdeposition. Preferably, the spacing between the first and the secondgrids 118, 120 ranges from approximately 3 cm to 7 cm. More preferably,the spacing between the first and the second grids 118, 120 ranges fromapproximately 4 cm to 6 cm. In one embodiment, the spacing between thefirst and the second grids 118, 120 is approximately 5 cm. Preferably,the spacing between the first grid 118 and the substrate 104 ranges fromapproximately 2 cm to 5 cm. More preferably, the spacing between thefirst grid 118 and the substrate 104 ranges from approximately 2 cm to 3cm. In one embodiment, the spacing between the first grid 118 and thesubstrate 104 is approximately 3 cm.

[0036] Electrically, the first and the second grids 118, 120 can beconnected and biased through the second wire 124 and the second powersupply. In one embodiment, the voltage range for the biasing of thefirst and the second grids 118, 120 is approximately −300 to −350 Voltsdirect current (Vdc) relative to ground. In another embodiment, thefirst and the second grids 118, 120 are not shorted together, but areconnected to their own power supplies. Although smaller voltages can beused, the relatively large voltage range of −300 to −350 Vdcadvantageously reduces the coating of the first and the second grids118, 120 during the deposition process, which would otherwise reduce theincorporation of the Li and/or Mg in the ferroelectric film.

[0037] The insulator 108 isolates the tray 106 and the substrate 104from ground potential. One embodiment of the insulator 108 is fabricatedfrom a quartz plate. The insulator 108 permits biasing of the substrate106 to a potential other than ground through the first conductor 114 andthe first power supply 112. In one embodiment, the voltage range for thebiasing of the substrate is approximately 0 to −50 Vdc. Of course, thesubstrate 104 and the tray 106 can simply be grounded without theinsulator 108 when 0 Vdc is selected as the biasing of the substrate.

[0038]FIG. 2 illustrates a portion of the wire screen 200 according toone embodiment of the first and the second grids 118, 120. To reduceedge effects, the first and the second grids 118, 120 are preferablycircular in shape and larger in diameter than the diameter of thesubstrate 104. For example, where the substrate 104 is 300 mm indiameter, the first and the second grids 118, 120 can be 35 cm indiameter.

[0039] The portion of the wire screen 200 shown illustrates interwovenwires 210, 220. The diameter of the wire, represented by A in FIG. 2,and the spacing between wires, represented by B and C in FIG. 2, alsovaries the rate of deposition. The spacing between wires preferablyranges from 3 mm to 5 mm, and more preferably from 4 mm to 5 mm. Thewire diameter ranges preferably from 0.5 mm to 1.2 mm, and morepreferably from 1.0 mm to 1.2 mm. In one embodiment, for processing300-mm wafers, the diameter of the wire, A, is in the range ofapproximately 0.5 to 1.0 mm and the spacing between the wires, B and C,is about 5 mm.

[0040] The wires are made from the desired dopant or dopants,specifically Li, Mg, or a combination of the two in the illustratedembodiment. Advantageously, the Li and/or Mg for the wires can beproduced by a relatively high purity process, such as a zone-refinedprocess. Material from the wires is deposited on the substrate 104together with the ZnO as dopants to the ZnO to produce the ferroelectricfilm. Where the first and the second grids 118, 120 include both Li andMg, the ratio of the Li to Mg in the first and the second grids 118, 120controls the ratio of the Li to Mg doping of the ferroelectric film. Theferroelectric properties of the ferroelectric film can thereby beconveniently tailored by selecting the desired ratio of Li to Mg in thefirst and the second grids, 1 18, 120.

[0041] The resulting film exhibits ferroelectric properties. In oneembodiment, the ferroelectric film is characterized by the generalformula of Zn_(x)(Li_(y)Mg_(z))O, where x preferably ranges fromapproximately 0.70 to 0.99, y and z can independently range fromapproximately 0.00 to 0.30, and x, y, and z substantially sum to 1. Morepreferably, x ranges from approximately 0.80 to 0.95, y ranges fromapproximately 0.01 to 0.05, z ranges from approximately 0.04 to 0. 15,and x, y, and z again substantially sum to 1. In one embodiment, x isabout 0.9, y is about 0.02, and z is about 0.08.

[0042] Embodiments of the present invention advantageously permit thefabrication of ferroelectric films on large substrates with high puritycomponents and at relatively easily varied and controlled conditions.The control over the process allows for relatively high yields, whichcan lower production costs. A large substrate further allows moredevices to be fabricated at the same time, further reducing overallproduction costs.

[0043] Various embodiments of the present invention have been describedabove. Although this invention has been described with reference tothese specific embodiments, the descriptions are intended to beillustrative of the invention and are not intended to be limiting.Various modifications and applications may occur to those skilled in theart without departing from the true spirit and scope of the invention asdefined in the appended claims.

We claim:
 1. A consumable control grid that supplies a dopant in aplasma deposition process for producing ferroelectric films, the controlgrid comprising: an electrical connection adapted to allow the controlgrid to sustain an electrical bias; a substantially planar conductorwherein a material comprising a surface of the substantially planarconductor supplies the dopant in the plasma deposition process forproducing ferroelectric films; and openings defined within the conductoradapted to allow the passage of plasma products.
 2. The consumablecontrol grid as defined in claim 1, wherein the substantially planarconductor corresponds to a wire screen, and where the openings definedwithin the conductor corresponds to spacing between wires of the wirescreen.
 3. The consumable control grid as defined in claim 1, whereinthe material includes a zone refined material.
 4. A plasma assistedchemical vapor deposition system, which deposits a ferroelectric film ona substrate, the system comprising: a heater adapted to heat thesubstrate to a first temperature; a first mass flow controller forintroducing zinc to an interior of an electromagnetic excitationchamber; a second mass flow controller for introducing oxygen to theinterior of the electromagnetic excitation chamber; means for ionizingthe zinc and the oxygen to form a plasma with microwave power; and atleast one control grid positioned in proximity to the substrate, wherethe control grid is further composed of a source material for theferroelectric film, and where the control grid is biased to a negativeelectric potential.
 5. The system as defined in claim 4, furthercomprising means for propelling the plasma towards the substrate with amagnetic field.